The invention relates to a steering wheel provided with an air bag device and, more particularly, to a steering wheel in which horn switch mechanisms are arranged between the air bag device and a steering wheel body.
In the prior art, a steering wheel provided with an air bag device and having horn switch mechanisms arranged respectively between a mandrel of a steering wheel body and a bag holder of the air bag device has been known from Japanese Patent Unexamined Publication No. 3-79471 (79471/1991).
The steering wheel disclosed in the above-described publication is such that either of upper and lower contact pieces which form the horn switch mechanisms, that is, the lower contact pieces formed by the mandrel of the steering wheel body, and the upper contact pieces and the bag holders of the air bag device are connected to each other by mounting screws.
The upper contact pieces connected to the bag holder are biased upwardly by two coil springs, which are interposed between the lower contact pieces and the upper contact pieces, and spaced from the lower contact pieces. A distance of the spacing between the upper and lower contact pieces is restricted by restriction screws which are threaded into the lower contact pieces.
The coil springs and the restriction screws are arranged such that the two contact pieces are prevented from being energized by an insulation member. Further, the pairs of upper and lower contact pieces are arranged respectively on right and left sides of the steering wheel.
In the steering wheel disclosed in the aforesaid publication, when the horn switch mechanisms are to be operated, a pad of the air bag device is pushed down to cause the upper contact pieces, which are connected to the bag holder of the air bag device by the mounting screws, to be moved downwardly against biasing forces of the coil springs, so that electrical contacts thereof come into contact with electrical contacts of the lower contact pieces to operate the horn.
However, in the steering wheel disclosed in the aforementioned publication, when the air bag of the air bag device expands, the bag holder is moved downwardly at an initial stage under the influence of a pressure upon expansion. At the time of completion of expansion, the bag holder will be moved upwardly by an inertia force produced by the expansion of the air bag.
Specifically, at the beginning of the expansion of the air bag, the bag holder is moved downwardly with the result that the upper contact pieces come into contact with the contacts of the lower contact pieces. Since the lower contact pieces are provided by the mandrel, it is possible to easily ensure a structure having a strength which sufficiently opposes the pressure by which the bag holder is moved.
However, at the time of completion of the expansion of the air bag, the bag holder will move upwardly to produce a force which acts upon the upper contact pieces, causing the upper contact pieces to separate from the restriction screws (a force opposed against the restriction screws). Thus, the upper contact pieces are required to be of high strength.
Thus, in the prior-art steering wheel, a strength of the upper contact pieces per se, the strength of the restriction screws per se which restrict upward movement of the upper contact pieces and a mounting strength, with which the restriction screws are threadedly engaged with the lower contact pieces must be ensured. This disadvantageously limits the degree of freedom in the design of the steering wheel mechanism.